Casey, who had shot a six-under-par 66 on Thursday, came unstuck with a disastrous triple bogey eight on the 14th after teeing off on the 10th hole.
One shot back. Jamie Lovemark.
On Saturday, the two-time major victor played like he should have been. He was coasting along Friday, playing par golf and hanging out at the top of the leaderboard. But then he three-putted from long range on the 13th and the 14th, and lost all hope when he missed the green on the 17th while going after the flag. I missed another one on four from about three feet, but I didn't feel like I was hitting bad putts, I was just misreading them. It doesn't need to be too much of a change for this golf course to play pretty hard. Of the group, it was Casey who had the wildest day. No harm, no foul.
None of the leaders had as shaky a stretch as Casey, who made 8 on the par-5 14th hole - after hitting into the fescue and almost whiffing on his swing to get out - and bogeyed the next hole.
The world number 14 found himself in trouble when he flew his approach out of rough over the green into the deep fescue and needed five more strokes to get in the cup.
McIlroy was never in the short grass from the 11th hole to the end on Thursday when he opened with a career-worst 78, and he didn't do almost enough in the second round until it was far too late. So there he stood at two under and seemingly out of the picture.
Casey birdied five straight holes before a 32-foot putt for birdie No. 6 strayed a foot left of the hole at the par-4 fourth.
A rattled Casey added another bogey at the 15th to extend his miserable run before hitting the turbo-charger and firing off five consecutive birdies.
"Not every day you enjoy a round of golf with an eight on the card, but I'm a pretty happy man", Casey said after his one-under round. "But I played pretty well. But you don't really know until you put a card in your hand and you're under the gun a little bit", said McIlroy, who said goodbye at 5 over. You can make a bogey kind of anywhere ...
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It's not been a good tournament for Australians, with Marc Leishman - who is four shots off the lead after an even-par 72 - set to be the only Aussie to make the cut.
With two rounds to play there is some 23 players within four shots of the lead.
Matsuyama is playing in his fifth U.S. Open.
It was the second consecutive US event the Briton has withdrawn from having pulled out of last month's Players Championship in the second round.
Kim won The Players Championship in May for his second PGA Tour title.
The 117th U.S. Open is getting good.
"It is going to be great as I've never done this before", he said looking ahead to the weekend.
"I'm optimistic with where my game is", he said. "For me, if I can just stay where I am, just keep doing what I'm doing, I'll have a chance".
NOTES: Japan's Hideki Matsuyama and Chez Reavie each posted a 7-under 65, which represented the day's lowest scores a day after Fowler posted the same score. But on Sunday, the winds, which so far have been mild, are expected to be in a force, making the final round a real test of skill.